Swimmers in surf observed by shark at Northland beach Whananaki North

Swimmers splashing in the surf at a Northland beach were unaware of a shark lurking just behind the breakers, only metres away.

But avid aviator Rusty Russell had a bird's-eye view of just how close the oblivious humans were to the dark, lithe, shape cruising the crystal clear waters near Whananaki North.

Mr Russell, a senior instructor with the Whangarei Flying Club, was flying above the beach off Barron's farm on Sunday afternoon when he spotted the dark shape of the shark which stood out against the white sandy background.

"There were about five people in the surf and they would have had no idea the shark was there.

"I reckon it was between 3 and 3.5m long but I'm no shark expert so I don't know what sort it was," Mr Russell said.

A frequent flyer in the skies above Northland Mr Russell said it was not uncommon to see sharks close to shore.

"We always see sharks around and they are always around people."

He had been flying with two other aircraft which landed on the public airstrip at Whananaki North. The group went to the local store for an icecream before taking off and seeing the shark on the return flight to Whangarei Airport.

Shark expert and Department of Conservation marine scientist Clinton Duffy said that at the start of summer the right conditions would mean more reports of shark activity.

"There probably won't be any more sharks than there normally are, they will just be more visible if we get calm weather."

Mr Duffy said people could expect sightings of many sub-tropical species, as well as great whites because there was a semi-resident population in New Zealand.

"They spend most of the summer here, arriving in November/December, before the adults leave in July/September."

He noted "high-risk" places included out the back of the surf break on beaches, areas near deep channels, around concentrations of bait fish, and near seal colonies.

About 70 species of sharks are found in New Zealand's waters. They range from the tiny pygmy shark to giant whale sharks. In between are several dangerous species including the great white and mako sharks.